1. Field of the Invention
This invention relates to electrochemical systems such as zinc-chlorine, including a chlorine positive electrode, and to an improved technique for reversibly storing chlorine in such systems.
2. Description of the Prior Art
Conceptually, the zinc-chlorine cell can be considered to be among the simplest of electrochemical systems in that only zinc, chlorine and zinc chloride actively participate in the basic electrochemical reaction. In its most elementary form, a rechargeable zinc-chlorine cell could be constructed with two inert, electronically conductive electrode substrates in an aqueous solution of zinc chloride. A charge/discharge cycle would thus involve the deposition and the subsequent anodic dissolution of zinc at the negative electrode and the liberation of chlorine with its subsequent cathodic reduction at the positive electrode. Moreover, and importantly, this system potentially can provide relatively high energy densities.
While such a battery is inherently simple from the electrochemical standpoint, chlorine gas, which is highly toxic and corrosive, is liberated during the charge cycle and is difficult to handle. Further, and importantly, the chlorine being formed in the charge cycle must be stored in some fashion to avoid severe performance penalties due to coulombic inefficiencies. However, the chlorine stored during charging must be made available during the discharge cycle to provide satisfactory voltaic efficiencies.
A major problem in the design of cells of this type has been to provide an efficient means of chlorine storage while providing, during discharge, adequate chlorine availability. One early attempt at storing chlorine in a zinc-chlorine system is described in U.S. Pat. No. 1,377,722. Chlorine generated during charging is stored as a liquid by introducing a gas such as air or hydrogen to pressurize the container to the desired level (e.g.--on the order of 20 atmospheres) to maintain chlorine in the liquid phase. In addition to requiring relatively expensive container materials, the complexity of the system is considered undesirable.
More recently, U.S. Pat. No. 3,713,888 describes a method for storing halogens in a battery system by employing halogen hydrates. The system described therein utilizes a circulating system wherein electrolyte is circulated through an electrode area and a hydrate of the halogen given off during charge is formed. The halogen hydrate, which is a solid, is subsequently stored in a separate storage area. This system involves relatively significant energy requirements due to the temperatures needed to maintain the solid halogen hydrate, thus inherently providing a relatively low energy efficiency for the system. It is further believed that this system may not provide the reliability desired for large cells or batteries, such as may be employed in load leveling applications.
Even more recently, U.S. Pat. Nos. 4,038,459 and 4,038,460 disclose a large number of compounds which may be added to the electrolyte of halogen cells to complex the halogen. U.S. Pat. No. 4,038,459 thus discloses various alcohols and nitriles which form insoluble oil-like complexes with halogens. U.S. Pat. No. 4,038,460 shows various halogen complexing ethers which likewise form insoluble, oil-like complexes with the halogens. A serious disadvantage of these alcohols, ethers, and nitriles is their instability. Such complexes tend to degrade rather rapidly in the cell environment. This approach is accordingly considered undesirable.
Thus, despite the potential of this system, as far as is known, little if any use has been made commercially. This is believed due, at least in major part, to the chlorine storage problem. Accordingly, despite the considerable efforts made over the years to provide an adequate solution, there still exists the need to provide a means of chlorine storage capable of achieving acceptable electrochemical performance characteristics in an efficient, reliable and adequately simple system.
Further, it is known that certain compounds will combine with chlorine to form chlorinated derivatives, often generally referred to as pseudo-halogens, typically having an N--Cl bond. While most of these compounds are believed to be solids, some are liquids, such as, for example, dichlorourethane. A substantial amount of literature has been generated, directed generally to the chemistry of such compounds.
Various pseudo-halogens have been utilized for many commercial applications. Pseudo-halogens have thus been suggested as a source of active chlorine, such as, for example, for disinfectant and/or sterilization purposes as well as in bleaching compositions. Generally, such use requires a relatively slow release of active chlorine; and, it is believed, that most, if not all, of the materials used heretofore were solids.
It has also been suggested to use pseudo-halogens in primary cells. For example, U.S. Pat. No. 2,874,205 describes a cell composed essentially of a high-magnesium alloy anode and a cathode material of which the key component is an organic oxidizing substance containing a chemically combined positive halogen. Suitable organic substances includes amines, amides, amides of dibasic acids or quinones, cyclic ureides, carbonic acid amidines and pyrrolidinones. Likewise, C. K. Morehouse and R. Glicksman, N-halogen Organic Compounds as Cathode Materials for Primary Batteries, J. Electrochem. Soc., 467-473 (1957) discuss the electrochemical characteristics of N-halogen organic compounds, including, for example, N-chlorosuccinimide, N,N'-dichlorobiuret, N,N'-dichlorodimethylhydantoin and trichloromelamine. It is believed that these pseudo-halogens were considered to be solid in form.
Accordingly, despite the body of literature generated over the years and the need to provide a satisfactory solution to the chlorine storage problem, liquid pseudo-halogens, as far as is known, have not ever been proposed as a solution to such problem.
It is accordingly a principal object of the present invention to provide an electrochemical system in which chlorine storage may be achieved in an efficient, reliable, and relatively simple fashion. A related object is to provide such a system capable of achieving acceptable electrochemical performance characteristics.
A further object provides such a system in which chlorine storage is achieved without any substantial efficiency penalty.
Yet another object of this invention lies in the provision of a system not involving high pressure conditions. A related and more specific object provides a system which minimizes toxicity and corrosion considerations.
A still further object is to provide such a system in which chlorine may be stored for extended periods of time without the occurrence of any significant degradation.
Another object of the present invention provides such a system characterized by substantial flexibility, allowing the chlorine storage in such a system to be tailored to the requirements of the intended application.
Other objects and advantages of the present invention will become apparent from the following description and from the sole FIGURE which illustrates an exemplary cell construction.
While the invention is susceptible of various modifications and alternative forms, specific embodiments thereof are described in detail herein. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but, on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as expressed in the appended claims. Thus, for example, while the present invention will be principally described in connection with the illustrated zinc-chlorine rechargeable cell, it should be appreciated that the present invention may likewise be employed in other aqueous primary and secondary cells. Indeed, the present invention may be suitably utilized, as an illustration, in various redox systems, for example, the Cr.sup.2+, Cr.sup.3+ .parallel.Cl.sup.- /Cl.sub.2 system.